NEW YORK (Reuters) - Merck & Co said its experimental hepatitis C treatment met the main effectiveness goals of two late-stage studies and it expects to seek approval for the high-profile medicine by the end of the year.

But a much higher percentage of patients taking the Merck drug, boceprevir, compared with those taking standard treatments, dropped out of one of the trials due to adverse events, including anemia.

In the two trials, 66% of hepatitis C patients who took boceprevir plus standard drugs for a full 48 weeks had a sustained virologic response, significantly more than those who took standard treatment alone. But that compares with a 75% cure rate seen in a separate trial of a rival drug being developed by Vertex Pharmaceuticals Inc.

"We have a compelling profile with boceprevir, with impressive (cure) rates across both studies," Merck research chief Peter Kim said in an interview.

Boceprevir and Vertex's telaprevir are considered possible blockbuster products because of their potential to cure far more patients and in as little as half the time of standard drugs that require almost a year of treatment and often cause flu-like symptoms that are tough to tolerate.

Merck said it would provide more detailed clinical trial data at a meeting of the American Association for the Study of Liver Diseases that begins Oct. 29 in Boston.

The new class of drugs, which are combined with standard treatments, work against the hepatitis C virus by blocking a protease that the virus requires to replicate.

Merck said boceprevir, taken in combination with the company's Pegintron brand of interferon and ribavirin, significantly increased the number of patients who achieved a sustained virologic response, defined as no detectable virus levels 24 weeks after the end of treatment -- compared with those who received the standard drugs plus a placebo.

One of the trials, called HCV RESPOND-2, involved 403 patients with genotype 1, the most common form of hepatitis C, who had failed prior therapy with interferon and ribavirin. The other trial, called HCV SPRINT-2, enrolled 1,097 patients with genotype 1 who had not previously been treated for the virus.

In both trials, a significant number of patients received 48 full weeks of treatment. But patients with undetectable virus at week 8 and again at certain points later in the studies were able to stop all treatment at 36 weeks in the smaller trial, and at 28 weeks in the larger study.

In the HCV RESPOND-2 study, 66% of those receiving boceprevir for 48 weeks were cured, while cures were seen in 59% of those receiving shorter treatment regimens of the medicine. That compared with a 21% cure rate for those receiving standard treatments.

In the HCV SPRINT-2 study of previously untreated patients, 66% of those receiving boceprevir for 48 weeks were cured, along with 63% of those on shorter regimens. Cures were seen in 38% of those receiving standard therapy.

Vertex is expected next month to unveil data from another late-stage trial of telaprevir in tougher-to-treat patients who had failed prior treatment with standard drugs.

Industry analyst Tim Anderson from Sanford Bernstein said available data from separate trials of boceprevir and telaprevir suggest the Merck drug is less effective.

Moreover, he said boceprevir seems more likely to cause anemia. The question is whether the need for an additional anemia drug on top of the three-drug regimen will greatly discourage use of boceprevir, should it be approved.